Isotopic gas analyzer and method of judging absorption capacity of carbon dioxide absornemt

ABSTRACT

In an isotopic gas analyzer, a gas injector ( 21 ) is provided for pressurizing a gas specimen in cells ( 11   a,   11   b ). The pressurization of the gas specimen virtually produces the same effect as increasing the concentration of carbon dioxide in the gas specimen, thereby improving an S/N ratio for the analysis and hence data reproducibility.

TECHNICAL FIELD

[0001] Isotopic analyses are useful for diagnosis of diseases in medicalapplications, in which the metabolic functions of a living body can bedetermined by administering an isotope-containing drug to the livingbody an d then detecting a change in the concentration ratio of theisotope.

[0002] The present invention relates to a stable isotope measurementmethod for spectrometrically analyzing an isotopic gas for determiningthe isotopic gas concentration ratio on the basis of a difference inlight absorption characteristic between isotopes.

BACKGROUND ART

[0003] Bacteria called Helicobacter Pylori (HP) are generally knownwhich cause gastric ulcers and gastritis.

[0004] If HP is present in the stomach of a patient, an antibioticshould be administered to the patient for bacteria removal treatment.Therefore, it is indispensable to check if the patient has HP. HP has ahigh urease activity for decomposing urea into carbon dioxide andammonia.

[0005] Carbon has isotopes having mass numbers of 12, 13 and 14, amongwhich the isotope ¹³C having a mass number of 13 is easy to handlebecause of its non-radioactivity and stability.

[0006] If the concentration of ¹³CO₂ as a final metabolic product inbreath of the patient, more specifically, a ¹³CO₂/¹²CO₂ concentrationratio, can successfully be determined after ¹³C-labeled urea isadministered to the patient, the presence of HP can be confirmed.

[0007] However, the ¹³CO₂/¹²CO₂ concentration ratio in naturallyoccurring carbon dioxide is 1:100, making it difficult to accuratelydetermine the concentration ratio in the breath of the patient.

[0008] There have conventionally been known methods for determining a¹³CO₂/¹²CO₂ concentration ratio by way of infrared spectrophotometry(see Japanese Examined Patent Publications No. 61-42249 (1986) and No.61-42220(1986)).

[0009] The method disclosed in Japanese Examined Patent Publication No.61-42220 employs two cells respectively having a long path and a shortpath. The path lengths of the cells are adjusted so that a ¹³CO₂absorbance in one of the cells is equalized with a ¹²CO₂ absorbance inthe other cell. Light beams respectively having wavelengths suitable fordetermination of the ¹³CO₂ absorbance and the ¹²CO₂ absorbance areapplied to the respective cells, and the intensities of transmittedlight beams are measured. According to this method, an absorbance ratiofor the concentration ratio in naturally occurring carbon dioxide can beset at 1. Therefore, the absorbance ratio is changed correspondingly toa change in the concentration ratio. This allows for detection of thechange in the concentration ratio.

[0010] (A) Even if the methods employing the infrared spectrophotometryare used, it is difficult to detect a slight change in the concentrationratio. The sensitivity can be enhanced by using longer cells, but theuse of the longer cells increases the size of the isotopic gas analyzer.

[0011] Another approach is to provide mirrors at opposite ends of thecells for reflecting the light beams many times. However, the cells eachhave a greater volume, so that the isotopic gas analyzer has acorrespondingly greater size.

[0012] It is therefore an object of the present invention to provide astable isotope measurement method, which can determine theconcentrations of component gases with a satisfactory measurementreproducibility and with a higher measurement accuracy by introducing agas specimen containing carbon dioxide ¹³CO₂ and carbon dioxide ¹²CO₂ asthe component gases into cells, measuring the intensities of light beamstransmitted through the cells at wavelengths suitable for analysis ofthe respective component gases, and processing data indicative of thelight intensities, and yet is free from a size increase.

[0013] (B) In the methods employing the infrared spectrophotometry, areference gas having a CO₂ concentration of zero, i.e., air havingpassed through a carbon dioxide absorbent, is filled in the cells, and areference absorbance measuring process is preliminarily performed foraccurate measurement of the absorbances of ¹²CO₂ and ¹³CO₂.

[0014] Where the carbon dioxide absorbent is used as described above,the carbon dioxide absorbent is gradually deteriorated, and it isdifficult to determine when the absorbent needs replacement.

[0015] The replacement time may be indicated on the basis of the numberof times of the analysis, or determined on the basis of a change in thecolor of the carbon dioxide absorbent which is adapted to be colored bya reaction with carbon dioxide.

[0016] Where the determination of the replacement time is based on thenumber of the times of the analysis, however, the analysis may sufferfrom an error which occurs due to variations in the absorption capacityof the carbon dioxide absorbent depending on production lots.

[0017] Where the carbon dioxide absorbent variable in color is used, thecolor of the absorbent returns to its original color when the air flowis stopped. Therefore, it is difficult to determine the replacementtime.

[0018] It is therefore another object of the present invention toprovide a method of judging the absorption capacity of a carbon dioxideabsorbent, which can accurately indicate a replacement time of thecarbon dioxide absorbent by quantizing the degree of the deteriorationof the carbon dioxide absorbent.

SUMMARY OF THE INVENTION

[0019] (A) The stable isotope measurement method according to thepresent invention pressurizes a gas specimen in the cell, measures anabsorbance of the component gases, and determines a concentration ratioof the component gases on the basis of a calibration curve.

[0020] The pressurization of the gas specimen virtually produces thesame effect as increasing the carbon dioxide concentration in the gasspecimen, thereby improving an S/N ratio and hence the measurementaccuracy and the measurement reproducibility without the need forincreasing the lengths of the cells. Further, the size increase of theanalyzer can be obviated.

[0021] Where the internal pressures of the cells are increased to 2 atmby the pressurization, a sufficient effect can be provided (see anembodiment to be described later).

[0022] (B) The method of judging the absorption capacity of the carbongas absorbent according to the present invention comprises the steps of:performing a first light intensity measuring process by introducing airhaving passed through a vessel containing the carbon dioxide absorbentinto the cells; performing a second light intensity measuring process byintroducing air not having passed through the vessel containing thecarbon dioxide absorbent into the cells; and judging the absorptioncapacity of the carbon dioxide absorbent on the basis of a lightintensity measured in the first light intensity measuring step and alight intensity measured in the second light intensity measuring step.

[0023] With this arrangement, the air having passed through the vesselcontaining the carbon dioxide absorbent and the air not having passedthrough the vessel containing the carbon dioxide absorbent arerespectively optically analyzed to determine how much carbon dioxide isabsorbed by the carbon dioxide absorbent by comparing the air havingpassed through the vessel with the air not having passed through thevessel.

[0024] In the judgment method, the ratio of the light intensity measuredin the first light intensity measuring step to the light intensitymeasured in the second light intensity measuring step is compared with athreshold for judgment of the absorption capacity of the carbon dioxideabsorbent.

[0025] In accordance with the present invention, variations in thejudgment among individuals can be eliminated. Further, the carbondioxide absorbent can be used up to its capacity, allowing for highlyreliable isotopic gas spectrophotometric analysis. Further, variationsin the absorption capacity of the carbon dioxide absorbent depending onproduction lots do not affect the isotopic gas spectrophotometricanalysis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a block diagram illustrating the overall construction ofan isotopic gas spectrophotometric analyzer;

[0027]FIG. 2(a) is a plan view illustrating a gas injector 21 forquantitatively injecting a gas specimen;

[0028]FIG. 2(b) is a front view illustrating the gas injector 21;

[0029]FIG. 3 is a diagram illustrating a gas flow path to be employedwhen the gas flow path and a cell chamber 11 are cleaned with a cleanreference gas;

[0030]FIG. 4 is diagram illustrating a gas flow path to be employed whena light intensity measuring process is performed on the reference gas;

[0031]FIG. 5 is a diagram illustrating a gas flow path to be employedwhen a base gas is sucked into the gas injector 21 from a breathsampling bag;

[0032]FIG. 6 is a diagram illustrating a gas flow path to be employedwhen a part of the base gas is mechanically ejected from the gasinjector 21 to supply the base gas into a first sample cell 11 a and asecond sample cell 11 b;

[0033]FIG. 7 is a diagram illustrating a gas flow path to be employedwhen the rest of the base gas is completely ejected from a cylinder 21 bwith a valve V6 being closed;

[0034]FIG. 8 is a diagram illustrating a gas flow path to be employedwhen air for sample gas dilution is sucked in;

[0035]Fig. 9 is a diagram illustrating a gas flow path to be employedwhen a sample gas is sucked in to the gas injector 21 from anotherbreath sampling bag;

[0036]FIG. 10 is a diagram illustrating a gas flow path to be employedwhen the sample gas is supplied into the first sample cell 11 a and thesecond sample cell 11 b;

[0037]FIG. 11 is a diagram illustrating a gas flow path to be employedwhen the sample gas is pressurized in the first sample cell 11 a and thesecond sample cell 11 b with the valve V6 being closed;

[0038]FIG. 12 is a diagram illustrating a gas flow path to be employedwhen air is sucked into the cylinder 21 b;

[0039]FIG. 13 is a diagram illustrating a gas flow path to be employedwhen the air is ejected at a constant flow rate from the cylinder 21 bfor the light intensity measuring process;

[0040]FIG. 14 is a diagram illustrating a gas flow path to be employedwhen the reference gas is sucked into the gas injector 21;

[0041]FIG. 15 is a diagram illustrating a gas flow path to be employedwhen the reference gas is filled in the first sample cell 11 a and thesecond sample cell 11 b with the use of the gas injector 21;

[0042]FIG. 16 is a graph illustrating a relationship between anadditionally injected amount (pressurization degree) of the gas specimenand a standard deviation indicative of variations in Δ¹³C data;

[0043]FIG. 17 is a graph obtained by plotting a relationship between thetotal period of use of a carbon dioxide absorbent and an intensity ratio¹²Ratio; and

[0044]FIG. 18 is a graph obtained by plotting a relationship between thetotal period of the use of the carbon dioxide absorbent and a standarddeviation SD of Δ¹³C data indicative of changes Δ¹³C in ¹³C calculatedon the basis of a plurality of measurements.

BEST MODE FOR CARRYING OUT THE INVENTION

[0045] An embodiment of the present invention will hereinafter bedescribed in detail with reference to the attached drawings. In thisembodiment, a ¹³C-labeled urea diagnostic drug is administered to apatient, and then a ¹³CO₂ concentration in breath sampled from thepatient is spectrophotometrically analyzed.

[0046] I. Breath Test

[0047] First, breath of the patient is sampled in a breath sampling bagbefore the administration of the urea diagnostic drug. Then, the ureadiagnostic drug is orally administered to the patient and, after a lapseof about 20 minutes, breath of the patient is sampled in another breathsampling bag in the same manner as in the previous breath sampling.

[0048] The breath sampling bags obtained before and after the drugadministration are respectively attached to predetermined nozzles of anisotopic gas spectrophotometric analyzer, and an automatic analysis is.performed in the following manner.

[0049] II. Isotopic Gas Spectrophotometric Analyzer

[0050]FIG. 1 is a block diagram illustrating the overall construction ofthe isotopic gas spectrophotometric analyzer.

[0051] The breath sampling bag containing the breath obtained after thedrug administration (hereinafter referred to as “sample gas”) and thebreath sampling bag containing the breath obtained before the drugadministration (hereinafter referred to as “base gas”) are respectivelyattached to the nozzles N1 and N2. The nozzle N1 is connected to anelectromagnetic valve V2 (hereinafter referred to simply as “valve”)through a metal pipe (herein after referred to simply as “pipe”), whilethe nozzle N2 is connected to a valve V3 through a pipe. Further, a pipefor introducing air is connected to a valve V5.

[0052] A reference gas supplied from a reference gas supplying section30 (which will be described later) flows into three paths. The referencegas flowing into one of the paths is fed into an auxiliary cell 11 c,and the reference gas flowing into another of the paths flows into avalve V1. The reference gas flowing into the other path flows into alight source unit for regulation of the temperature of the light sourceunit.

[0053] The reference gas flowing into the auxiliary cell 11 c isdischarged into a cell chamber 10 from the auxiliary cell 11 c.

[0054] An outlet of the valve V1 is connected to one port of a three-wayvalve V4, and another port of the three-way valve V4 is connected to agas injector 21 for quantitatively injecting the sample gas or the basegas. The gas injector 21 is a syringe-like configuration having a pistonand a cylinder. The piston is driven by cooperation of a pulse motor, afeed screw coupled to the pulse motor and a nut fixed to the piston(which will be described later).

[0055] The other port of the three-way valve V4 is connected to a first:sample cell 11 a for measuring a ¹²Co₂ absorbance. Pipes extending fromthe valves V2, V3 and V5 join a pipe which connects the valve V1 and thethree-way valve V4.

[0056] The cell chamber 11 includes the first sample cell 11 a having asmall length for measuring the ¹²CO₂ absorbance, a second sample cell 11b having a great length for measuring a ¹³CO₂ absorbance, and theauxiliary cell 11 c through which the reference gas flows. The firstsample cell 11 a communicates with the second sample cell 11 b, so thatthe gas introduced into the first sample cell 11 a directly enters thesecond sample cell, 11 b and discharged through a valve V6. Thereference gas is introduced into the auxiliary cell 11 c.

[0057] The first sample cell 11 a has a volume of about 0.6 ml, and thesecond sample cell 11 b has a volume of about 12 ml. Specifically, thelength of the first sample cell 11 a is 13 mm, and the length of thesecond sample cell 11 b is 250 mm. The auxiliary cell 11 c has a lengthof 236 mm. Sapphire windows pervious to infrared radiation are providedon opposite end faces of the cell chamber 11. The cell chamber 11 isenclosed by a heat insulating material such as polystyrene foam (notshown).

[0058] A reference character L denotes the infrared light source unit.The infrared light source unit L includes two waveguides 23 a, 23 b forprojection of infrared light beams. The infrared light beams may begenerated in any manner. For example, a ceramic heater (surfacetemperature: 450° C.) or the like may be used. A rotary chopper 22 isprovided for blocking the infrared light beams on a predetermined cycle.

[0059] The in frared light beams projected from the infrared lightsource unit L respectively pass along a first light path L1 extendingthrough the first sample cell 11 a and the auxiliary cell 11 c and alonga second light path L2 extending through the second sample cell 11 b.(see FIG. 1).

[0060] A reference character D denotes an infrared detector fordetecting the infrared light beams having passed through the cells.

[0061] The infrared detector D has a first wavelength filter 24 a and afirst detection element 25 a provided in the first light path, and asecond wavelength filter 24 b and a second detection element 25 bprovided in the second light path.

[0062] The first wavelength filter 24 a is designed to transmit infraredradiation having a wavelength of about 4280 nm for the measurement ofthe ¹²CO₂ absorbance, while the second wavelength filter 24 b isdesigned to transmit infrared radiation having a wavelength of about4412 nm for the measurement of the ¹³CO₂ absorbance. The first detectionelement 25 a and the second detection element 25 b are adapted fordetection of the infrared light beams.

[0063] The first wave length filter 24 a, the first detection element 25a, the second wave length filter 24 b and the second detection element25 b are housed in a package 26 filled with an inert gas such as Ar.

[0064] The temperature of the entire infrared detector D is kept at aconstant level by a heater and a Peltier element, and the internaltemperatures of packages 26 a, 26 b are each kept at a low level by aPeltier element 27.

[0065] Fans 28, 29 are provided for ventilation in the isotopic gasspectrophotometric analyzer.

[0066] The reference gas supplying section 30 is annexed to a main bodyof the isotopic gas spectrophotometric analyzer for supplying air freedof CO₂. The reference gas supplying section 30 includes a dust filter31, a compressor 32, a moisture removing section 33, a dry filter 34, aflow meter 35 and a carbon dioxide absorbing section 36 which areconnected in series.

[0067] The carbon dioxide absorbing section 36 employs, for example,soda lime (a mixture of sodium hydroxide and calcium hydroxide) as acarbon dioxide absorbent.

[0068] FIGS. 2(a) and 2(b) are a plan view and a front view,respectively, illustrating the gas injector 21 for quantitativelyinjecting a gas specimen. The gas injector 21 functions as “pressurizingmeans”.

[0069] The gas injector 21 includes a base 21 a, a cylinder 21 bprovided on the base 21 a, a piston 21 c fitted in the cylinder 21 b, amovable nut 21 d provided below the base 21 a and coupled to the piston21 c, and a feed screw 21 e threadingly engaged with the nut 21 d, and apulse motor 21 f for rotating the feed screw 21 e.

[0070] The pulse motor 21 f is driven in a normal direction and areverse direction by a driver circuit not shown. When the feed screw 21e is rotated by the rotation of the pulse motor 21 f, the nut 21 d ismoved back and forth in accordance with the direction of the rotation ofthe screw. Thus, the piston 21 c is moved back and forth to a desiredposition. Therefore, the introduction and ejection of the gas specimeninto/from the cylinder 21 b can be controlled as desired.

[0071] III. Measuring Procedure

[0072] The measurement is achieved by performing a reference gasmeasurement process, a base gas measurement process, the reference gasmeasurement process, a sample gas measurement process, and the referencegas measurement process in this order. In FIGS. 3 to 11, gas flow pathsare hatched.

[0073] During the measurement, the reference gas constantly flowsthrough the auxiliary cell 11 c. The flow rate of the reference gas iskept at a constant level by the flow meter 35.

[0074] III-1. Reference Measurement Process

[0075] The clean reference gas is passed through a gas flow path and thecell chamber 11 of the isotopic gas spectrophotometric analyzer as shownin FIG. 3 to clean the gas flow path and the cell chamber 11. At thistime, the cylinder 21 b is also cleaned by moving back and forth thepiston 21 c.

[0076] Then, the reference gas is ejected from the cylinder 21 b asshown in FIG. 4, and light intensities are measured by means of therespective detection elements 25 a, 25 b.

[0077] The light intensities thus measured by the first and seconddetection elements 25 a and 25 b are represented by ¹²R1 and ¹³R1,respectively.

[0078] III-2. Base Gas Measurement Process

[0079] With the valve V1 being closed and two ports of the valve V4being open as shown in FIG. 5, the reference gas is prevented fromflowing into the first sample cell 11 a and the second sample cell 11 b.Then, the valve V2 is opened, and the base gas is sucked into the gasinjector 21 from the breath sampling bag.

[0080] After the suction of the base gas, a part of the base gas ismechanically ejected from the gas injector 21 with one port of the valveV4 and the valve V6 being open as shown in FIG. 6, whereby the firstsample cell 11 a and the second sample cell 11 b are filled with thebase gas.

[0081] Then, the valve V6 is closed as shown in FIG. 7, and the rest ofthe base gas is completely ejected from the cylinder 21 b. Thus, thebase gas pressure in the first sample cell 11 a and the second samplecell 11 b is increased. In FIG. 7, a gas flow path containing the higherpressure gas is cross-hatched.

[0082] In this pressurized state, light intensities are measured by therespective detection elements 25 a, 25 b.

[0083] The light intensities thus measured by the first and seconddetection elements 25 a and 25 b are represented by ¹²B and ¹³B,respectively.

[0084] III-3. Reference Measurement Process

[0085] The cleaning of the gas flow path and the cells and the lightintensity measurement for the reference gas are performed again (seeFIGS. 3 and 4).

[0086] Light intensities thus measured by the first and second detectionelements 25 a and 25 b are represented by ¹²R2 and ¹³R2, respectively.

[0087] III-4. Sample Gas Measurement Process

[0088] Air for sample gas dilution is sucked into the gas injector 21with the valve V5 being open as shown in FIG. 8. When the CO₂concentration in the sample gas is higher than the CO₂ concentration inthe base gas, the sample gas is diluted so that these CO₂ concentrationsare equalized with each other.

[0089] If the CO₂ concentration in the base gas is higher than the CO₂concentration in the sample gas, the base gas is diluted prior to thesuction of the base gas (see FIG. 5)

[0090] The CO₂ concentration in the base gas and the CO₂ concentrationin the sample gas are preliminarily determined through the lightintensity measurement by means of the detection elements 25 a, 25 b.

[0091] For detailed information on the dilution process, seeInternational Publication WO98/30888.

[0092] Then, the sample gas is sucked into the gas injector 21 from thebreath sampling bag with the reference gas being prevented from flowinginto the first sample cell 11 a and the second sample cell 11 b (seeFIG. 9). Thus, the sample gas is diluted in the cylinder 21 b.

[0093] After the suction of the sample gas, the first sample cell 11 aand the second sample cell 11 b are filled with the sample gas as shownin FIG. 10.

[0094] Then, the valve V6 is closed as shown in FIG. 11, and the samplegas is mechanically ejected from the gas injector 21, whereby the samplegas is pressurized in the first sample cell 11 a and the second samplecell 11 b.

[0095] The operation of the gas injector 21 is stopped, and then lightintensities are measured by the detection elements 25 a, 25 b.

[0096] The light intensities thus measured by the first and seconddetection elements 25 a and 25 b are represented by ¹²S and ¹³S,respectively.

[0097] III-5. Reference Measurement Process

[0098] The cleaning of the gas flow path and the cells and the lightintensity measurement for the reference gas are performed again (seeFIGS. 3 and 4).

[0099] Light intensities thus measured by the first and second detectionelements 25 a and 25 b are represented by ¹²R3 and ¹³R3, respectively.

[0100] IV. Data Processing

[0101] IV-1. Calculation of Base Gas Absorbances

[0102] The ¹²CO₂ absorbance ¹²Abs(B) and the ¹³CO₂ absorbance ¹³Abs(B)of the base gas are calculated on the basis of the transmitted lightintensities ¹²R1 and ¹³R1 for the reference gas, the transmitted lightintensities ¹²B and ¹³B for the base gas and the transmitted lightintensities ¹²R2 and ¹³R2 for the reference gas.

[0103] The ¹²CO₂ absorbance ¹²Abs(B) is calculated from the followingequation:

¹² Abs(B)=−log[2·¹² B/(¹² R1+¹² R2)]

[0104] The ¹³CO₂ absorbance ¹³Abs(B) is calculated from the followingequation:

¹³ Abs(B)=−log[2·¹³ B/(¹³ R1+¹³ R2)]

[0105] Since the calculation of the absorbances is based on the lightintensities obtained in the base gas measurement process and theaverages (R1+R2)/2 of the light intensities obtained in the referencemeasurement processes performed before and after the base gasmeasurement process, the influence of a drift (a time-related influenceon the measurement) can be eliminated. Therefore, there is no need forwaiting until the analyzer reaches a complete thermal equilibrium.(which usually takes several hours) at the start-up of the analyzer.Thus, the measurement can be started immediately after the start-up ofthe analyzer.

[0106] IV-2. Calculation of Sample Gas Absorbances

[0107] The ¹²CO₂absorbance ¹²Abs(S) and the ¹³CO₂ absorbance ¹³Abs(S) ofthe sample gas are calculated on the basis of the transmitted lightintensities ¹²R2 and ¹³R2 for the reference gas, the transmitted lightintensities ¹²S and ¹³S for the sample gas and the transmitted lightintensities ¹²R3 and ¹³R3 for the reference gas.

[0108] The ¹²CO₂ absorbance ¹²Abs(S) is calculated from the followingequation:

¹² Abs(S)=−log[2·¹² S/(¹² R2+¹² R3)]

[0109] The ¹³CO₂ absorbance ¹³Abs(S) is calculated from the followingequation:

¹³ Abs(S)=−log[2·¹³ S/(¹³ R2+¹³ R3)]

[0110] Since the calculation of the absorbances is based on the lightintensities obtained in the sample gas measurement process and theaverages of the light intensities obtained in the reference measurementprocesses performed before and after the sample gas measurement process,the influence of a drift can be eliminated.

[0111] IV-3. Calculation of Concentrations

[0112] The ¹²CO₂ concentration and the ¹³CO₂ concentration aredetermined with the use of a calibration curve. The calibration curve isprepared on the basis of measurement performed by using gas samples ofknown ¹²CO₂ concentrations and gas samples of known ¹³CO₂concentrations. Since the base gas and the sample gas are pressurizedduring the aforesaid measurement processes, these gas samples for thepreparation of the calibration curve are also pressurized during themeasurement.

[0113] For the preparation of the calibration curve, the ¹²CO₂absorbances for different ¹²CO₂ concentrations ranging from about 0% toabout 6% are measured. The ¹²CO₂ concentration and the ¹²CO₂ absorbanceare plotted as abscissa and ordinate, respectively, and the curve isdetermined by the method of least squares. An approximate quadraticcurve, which includes relatively small errors, is employed as thecalibration curve in this embodiment.

[0114] The ¹²CO₂ concentration and ¹³CO₂ concentration in the base gasand the ¹² CO₂ concentration and ¹³CO₂ concentration in the sample gasdetermined by using the aforesaid calibration curve are represented by¹²Conc(B), ¹³Conc(B), ¹²Conc(S) and ¹³Conc(S), respectively.

[0115] IV-4. Calculation of Concentration Ratios

[0116] The concentration ratio of ¹³CO₂ to ¹²CO₂ is determined. Theconcentration ratios in the base gas and in the sample gas are expressedas ¹³Conc(B)/¹²Conc(B) and ¹³Conc(S)/¹²Conc(S), respectively.

[0117] Alternatively, the concentration ratios may be defined as¹³Conc(B)/(¹²Conc(B)+¹³Conc(B)) and ¹³Conc(S)/(¹²Conc(S)+¹³Conc(S)).Since the ¹²CO₂ concentration is much higher than the ¹³CO₂concentration, the concentration ratios expressed in the former way andin the latter way are virtually the same.

[0118] IV-5. Determination of ¹³C change

[0119] A ¹³C difference between the sample gas and the base gas iscalculated from the following equation:

Δ¹³C=[(Concentration ratio in sample gas)−(Concentration ratio in basegas)]×10³/(Concentration ratio in base gas)

[0120] (Unit: per mil(per thousand))

[0121] V. Judgment of Absorption Capacity of Carbon Dioxide Absorbent

[0122] An explanation will next be given to a procedure for judging theabsorption capacity of the carbon dioxide absorbent. In FIGS. 12 to 15,gas flow paths are hatched.

[0123] During measurement, the reference gas is constantly passedthrough the auxiliary cell 11 c, and the flow rate of the reference gasis kept at a constant level by the flow meter 35.

[0124] V-1. Air Light Intensity Measurement Process

[0125] Air is sucked into the cylinder 21 b with the valve V1 beingclosed and the Valve V5 and two ports of the valve V4 being open asshown in FIG. 12.

[0126] The valve V4 is switched as shown in FIG. 13, and air is ejectedat a constant flow rate from the cylinder 21 b into the gas flow pathand the cell chamber 11 of the isotopic gas spectrophotometric analyzer.Then, a light intensity is measured by the detection element 25 a.

[0127] The light intensity thus measured by the first detection element25 a is represented by ¹²A.

[0128] V-2. Reference Gas Measurement Process

[0129] The reference gas is sucked into the gas injector 21 with thevalve V1 and two ports of the valve V4 being open as shown in FIG. 14.

[0130] After the suction of the base gas, the valve V4 is switched asshown in FIG. 15, and the base gas is mechanically ejected at a constantflow rate from the gas injector 21. Thus, the first sample cell 11 a andthe second sample cell 11 b are filled with the reference gas. In thisstate, a light intensity is measured by the detection element 25 a.

[0131] The light intensity thus measured by the first detection element25 a is represented by ¹²R.

[0132] V-3. Data Processing

[0133] A ¹²CO₂ intensity ratio ¹²Ratio is determined on the basis of thetransmitted light intensity ¹²A for air and the transmitted lightintensity ¹²R for the reference gas. The intensity ratio ¹²Ratio iscalculated from the following equation:

¹²Ratio=A/ ¹² R

[0134] As the intensity ratio ¹²Ratio approches 1, the absorptioncapacity of the carbon dioxide absorbent is reduced. More specifically,there is a relationship between the intensity ratio and the absorptioncapacity as shown in Table 1. TABLE 1 ¹²Ratio Absorption capacity 0.980100% 0.990  50% 1.000  0%

[0135] The absorption capacity of the carbon dioxide absorbent can bejudged on the basis of the thus determined intensity ratio ¹²Ratio withreference to Table 1.

[0136] When the intensity ratio ¹²Ratio is lower than a threshold (e.g.0.990), an indication of the deterioration of the carbon dioxideabsorbent is displayed on a liquid crystal display device (not shown) ofthe isotopic gas analyzer for information to a user. Further, theisotopic gas spectrophotometric analysis is not permitted until thecarbon dioxide absorbent is replaced.

EXAMPLE 1

[0137] Changes Δ¹³C were determined for a gas specimen having a ¹²CO₂concentration of 1% with the gas specimen being pressurized at aplurality of levels and without the pressurization of the gas specimen.

[0138] The gas specimen employed in this example was not a breath sampleof a patient as the sample gas or the base gas, but was air of 1% ¹²CO₂concentration contained in a single breath sampling bag having a greatersize. The breath sampling bag had two outlets, which were respectivelyconnected to the nozzles N1 and N2. Since the same gas specimen wasemployed for the measurement in this example, the changes Δ¹³C shouldhave normally been zero.

[0139] Table 2 shows the changes Δ¹³C calculated on the basis ofmeasurement results obtained when the measurement was performed tentimes by additionally injecting the gas in amounts of 0 ml (1 atm), 5 ml(about 1.25 atom), 10 ml (about 1.5 atm), 15 ml (about 1.75 atm) and 20ml (about 2 atm). TABLE 2 (%) Number of times of Additionally injectedamount (ml) measurement 0 5 10 15 20 1 0.6 1.3 0.9 0.1 −0.5 2 1.2 0.3−0.4 0.1 0.1 3 −0.5 0.9 0.1 0.4 0.0 4 0.0 −0.5 −0.2 −0.1 0.1 5 0.6 0.9−0.2 −0.5 −0.6 6 −0.8 −0.1 −0.1 −0.3 0.0 7 −0.6 0.1 0.9 −0.7 0.0 8 −0.40.4 −0.3 0.0 −0.1 9 0.6 0.0 0.6 0.1 −0.4 10  0.9 0.8 −0.1 −0.3 −0.3Average 0.16 0.41 0.12 −0.12 −0.17 Standard deviation 0.71 0.56 0.490.33 0.26 Maximum value 1.2 1.3 0.9 0.4 0.1 Minimum value −0.8 −0.5 −0.4−0.7 −0.6

[0140] A relationship between the additionally injected amount and astandard deviation indicative of variations in the Δ¹³C data is shown inFIG. 16.

[0141] As can be seen in FIG. 16, there was an obvious correlationbetween the additionally injected amount and the standard deviation. Asthe additionally injected amount (pressurization degree) increased, thestandard deviation was reduced.

[0142] Therefore, the pressurization effectively improves thereproducibility of the measurement data.

EXAMPLE 2

[0143] Soda lime (a mixture of sodium hydroxide and calcium hydroxide)was used as the carbon dioxide absorbent. Reactions are shown below.

CO₂+H₂O+2NaOH→Na₂CO₃+2H₂O

Na₂CO₃+Ca(OH)₂CaCO₃+2NaOH

[0144] The measurement was performed a plurality of times a day, and arelationship between the total period of. the use of the carbon dioxideabsorbent and the intensity ratio ¹²Ratio was plotted in a graph asshown in FIG. 17. As can be seen in FIG. 17, the intensity ratio ¹²Ratiosteeply increased when the total period exceeded about 300 hours.

[0145] In addition to the aforesaid measurement, measurement wasperformed by employing a reference gas prepared with the use of the samecarbon dioxide absorbent and a gas specimen having a ¹²CO₂ concentrationof 1% as the sample gas, and changes Δ¹³C in ¹³C were calculated. Thegas specimen employed in this example was not a breath sample of apatient as the sample gas or the base gas, but was air of 1% ¹²CO₂concentration contained inasingle breath sampling bag having a greatersize. The breath sampling bag had two outlets, which were respectivelyconnected to the nozzles N1 and N2.

[0146] More specifically, the ¹²CO₂ absorbance ¹²Abs and the ¹³CO₂absorbance ¹³Abs were respectively calculated from the followingequations:

¹² Abs=−log[¹² S/ ¹² R]

¹³ Abs=−log[¹³ S/ ¹³ R]

[0147] wherein ¹²S and ¹³S are transmitted light intensities for the gasspecimen, and ¹²R and ¹³R are transmitted light intensities for thereference gas. With the use of the calibration curve, a ¹²CO₂concentration ¹²Conc and a ¹³CO₂ concentration ¹³Conc were determined,and then a concentration ratio ¹³Conc/¹²Conc was calculated.

[0148] This procedure was performed again for the same gas specimen. Achange Δ¹³C was calculated from the following equation:

Δ¹³C=[(Concentration ratio at first time)−(Concentration ratio at secondtime)]×10³/(Concentration ratio at first time)

[0149] (Unit: per mil(per thousand))

[0150] The aforesaid procedure was repeated 10 times for calculation ofthe changes Δ¹³C.

[0151] Since the same gas specimen was employed in this example, thechanges Δ¹³C should have normally been zero.

[0152] However, there were deviations of measurement data from zero dueto measurement errors. Standard deviations SD were plotted in a graph asshown in FIG. 18.

[0153] As can be seen in FIG. 18, the standard deviation SD indicativeof variations in the measurement data exceeded 0.30 and steeplyincreased after the total use period reached 300 hours.

[0154] In the graph shown in FIG. 17, a total use period of 300 hourscorresponds to an intensity ratio ¹²Ratio of 0.99, which is a referencevalue to be employed as the threshold for the replacement of the carbondioxide absorbent. The value “0.99“ is merely an example, so that adifferent threshold may of course be employed depending on thespecifications of the analyzer.

1. (amended) a stable isotope measurement method for spectrometricallyanalyzing an isotopic gas by introducing a gas specimen containing aplurality of component gases into a cell, measuring intensities of lighttransmitted therethrough at wavelengths suitable for the respectivecomponent gases, and processing data of the light intensities todetermine a concentration ratio between the component gases, thecomponent gases being carbon dioxide ₁₂CO₂ and carbon dioxide ¹³CO₂, themethod comprising: a first step of introducing the gas specimen into thecell and into a gas injector which is communicated with the cell; asecond step of injecting the gas specimen by the gas injector in apredetermined amount in the cell and pressurizing the gas specimen inthe cell; a third step of determining absorbances of light transmittedtherethrough at the wavelengths suitable for the respective componentgases; and a fourth step of determining a concentration ratio betweenthe component gases in the gas specimen on the basis of a calibrationcurve prepared through measurement on pressurized gas samples eachcontaining the component gases in known concentrations.
 2. (amended) Astable isotope measurement method for spectrometrically analyzing anisotopic gas as set forth in claim 1, wherein the gas specimen ispressurized up to 2 atm in the cell.
 3. (deleted)
 4. A method of judgingthe absorption capacity of a carbon dioxide absorbent for use in anisotopic gas analyzing method for measuring the concentration of carbondioxide ¹³CO₂ in a gas specimen containing carbon dioxide ¹³CO₂ andcarbon dioxide ¹²CO₂ as component gases, the isotopic gas analyzingmethod comprising the steps of: introducing the gas specimen into cellsand measuring the intensities of light beams transmitted through thecells at wavelengths suitable for analysis of the respective componentgases; introducing air having passed through a vessel containing thecarbon dioxide absorbent as a reference gas into the cells and measuringthe intensities of light beams transmitted through the cells at thewavelengths suitable for the analysis of the respective component gases;and processing data indicative of the measurement results, characterizedby steps of: performing a first light intensity measuring process byintroducing air having passed through the vessel containing the carbondioxide absorbent into the cells; performing a second light intensitymeasuring process by introducing air not having passed through thevessel containing the carbon dioxide absorbent into the cells; andjudging the absorption capacity of the carbon dioxide absorbent on thebasis of a light intensity measured in the first light intensitymeasuring step and a light intensity measured in the second lightintensity measuring step.
 5. An absorption capacity judging method asset forth in claim 4, wherein the absorption capacity judging stepcomprises the step of comparing the ratio of the light. intensitymeasured in the first light intensity measuring step to the lightintensity measured in the second light intensity measuring step with athreshold.
 6. An absorption capacity judging method as set forth inclaim 4, wherein the light intensities are measured at a wavelengthsuitable for analysis of carbon dioxide ¹²CO₂ in the first and secondlight intensity measuring steps.